[ortac-monolith] declare abstract type when it is an alias for a core_type

[n-osborne]

type t = int
val f : t -> bool
(*@ b = f t *)

gives

module S = struct let t = declare_abstract_type ~var:"t" () end
let () = let spec = M.int ^!> bool in declare "f is Ok" spec R.f C.f
let () = let fuel = 100 in main fuel

where:

1. t is declared as an abstract type despite the fact that it is not
2. test declaration for f uses M.int where we would expect S.t